The present invention relates to a device at filters for filtering unwanted signals from radio transmitters and/or for use during combination of radio transmitters, wherein said filter has at lease one cavity for filtering radio signals, and wherein at least one compensating means is provided to compensate for deviations in frequency depending on expansion or contraction of the cavity due to expansion or contraction of the filter when said filter is heated or cooled during operation,
In order to limit the space the radio transmitter is occupying in the frequency spectrum, the radio signals must be filtered before they are transmitted into the ether. This is necessary since the modulation as well as imperfections in the power amplifier generate unwanted frequencies.
For carrying through the above function, prior art filters often have a cavity for filtering radio signals of or with one mode only.
This means that the filters often have many cavities, whereby the filter becomes very large and voluminous, causing high costs for material and production and requiring much space etc.
The object of the present invention is to provide a filter which eliminates this drawback and this is arrived at by providing the filter with the characterizing features of the subsequent claims.
By providing the filter with said characterizing features, it has been possible to make the filter substantially smaller and cheaper than previously. Also, the filter renders it easier to carry through customer adapted trimmings or alignments for removing also specific unwanted signals.